Generally, an aerostat refers to a system that remains aloft primarily using aerostatic buoyancy. Aerostats are known as flying machines. An example of an aerostat may be a large balloon filled with hot air or a buoyant gas (often hydrogen or helium). The earliest flights were made with hot air balloons using air heated with a flame, or hydrogen; later, helium was used. A balloon typically travels with the wind.
An aerostat's main structural component may be its envelope containing a lifting gas which is less dense than the surrounding atmosphere to provide buoyancy. An airship may refer to a type of aerostat or “lighter-than-air aircraft” that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. A typical airship carrying passengers, cargo, or both may move in the air from one location to another location. Currently, heavier than air structures having the length over 1,000 meters (m) may not be built due to certain structural limitations.